The present invention concerns the production of a fine-grained vinyl chloride suspension polymer and its use as a viscosity-lowering medium in polyvinyl chloride plastisol processing.
Herein, polyvinyl chloride plastisols or polyvinyl chloride pastes are understood to mean, in general, dispersions of fine polyvinyl chloride powders in plasticizers. Suitable paste-forming types of polyvinyl chloride include emulsion polymers and, in particular, also microsuspension polymers.
These polyvinyl chloride products are obtained after polymerization in latex form. They are processed generally by spray-drying into agglomerated secondary particles (plastisol grades). During plastisol processing, these agglomerates disintegrate predominantly into primary particles. The degree of such disintegration and the particle size distribution of the primary particles determine the flow properties of the paste.
Polyvinyl chloride pastes are utilized primarily as spread-coating pastes, dip-coating pastes, casting pastes, and molding pastes for a great variety of finished articles.
In general, pastes of low viscosities are desirable for the manufacture of finished articles low in plasticizer. It is known for this purpose to lower the paste viscosity by the addition of fine-grained suspension polyvinyl chloride products (so-called extender polyvinyl chloride). These are not paste-forming and are dissolved only at the gelling temperature.
According to German Pat. No. 1,645,668, it is possible to produce such suspension polymers for lowering the viscosity of dispersions of paste-forming polyvinyl chloride and plasticizers using methylhydroxypropylcelluloses having a viscosity of 50-500 m Pa s at 20.degree. C. in a 2% by weight solution, as the only suspension stabilizer, and using monomer-soluble catalysts.
However, as can be seen from Comparative Examples A and B herein, polymers produced in this way have a considerable proportion of coarse particles having a particle size&gt;100 .mu.m. This leads to an undesirable sedimentation of the coarse proportion and therefore causes difficulties during processing. Furthermore, the coarse proportion results in an undesirably rough surface, especially when applying very thin coatings. Also, the lowering in viscosity of the pastes by the addition of suspension polymers produced in this way fails to satisfy presently prevailing practical requirements.
As can be seen from Comparative Test C herein, very finely divided polyvinyl chloride polymers can be prepared by suspension polymerization in the presence of a suspension stabilizer and a tenside or emulsifier.
Because of its fineness and its minimum proportion of coarse particles&gt;100 .mu.m in size, a so-prepared polymer is suitable for thin coatings and does not tend toward sedimentation. However, due to its very high granular porosity, resulting from the addition of emulsifiers or tensides, the thus-manufactured product results in an inadequate lowering of paste viscosity.